1. Field of the Invention
The present invention relates to a carrier useful for a developer which can develop electrostatic latent images, and more particularly to a resin-coated carrier useful for an electrophotographic two-component developer and manufacturing method thereof.
2. Discussion of the Background
Among methods of developing electrostatic latent images using toner, a cascade developing method (U.S. Pat. No. 2,618,552) and a magnetic brush developing method (U.S. Pat. No. 2,874,063) are well known. Both of these developing methods use a two-component developer. A two-component developer includes a carrier which has relatively large particles and a toner which has relatively small particles which are retained on the surface of the carrier particles by electric forces, caused by friction between the toner particles and the carrier particles. When the two-component developer approaches an electrostatic latent image, a force that is caused by an electric field associated with the latent image and which attracts the toner particles toward the latent image becomes greater than the binding force of the toner particles and the carrier the toner particles are attracted to the latent image, resulting in visualization of the latent image. The developer is used repeatedly by being replenished with an amount of toner commensurate with the amount of toner spent for developing latent images. In this case, the toner particles should have a desired polarity and electric charge to be securely attracted to latent images. In addition, the carrier should continuously charge the toner particles as long as the carrier is used in the developer, so that the toner particles have the desired polarity and electric charge.
Further, a carrier that is used in the magnetic brush developing method should be appropriately magnetizable. Therefore, an iron powder carrier, a ferrite carrier, and a binder-type carrier which has resin particles including fine magnetizable particles are developed and utilized as a carrier for a magnetic brush developing method.
However, these carriers cause a problem in which toner particles adhere to the surface of the carrier when developing operations are repeatedly performed, resulting in a change of surface resistivity of the carrier, and, thereby, image qualities deteriorate (hereinafter referred to as the spent-carrier problem). Thus developers including these carriers have the problem of a short life.
In attempting to solve the spent-carrier problem, an iron powder carrier whose surface is coated with a resin is used, however the problem cannot be solved. This is because the iron carrier tends to deteriorate by receiving large stress due to its large true specific gravity when it is agitated in a developing unit. A ferrite carrier is spherical, and has a relatively small true specific gravity of from 4.5 to 5.5 and a relatively large bulk density of from 2 to 3 g/ml compared to the iron powder carrier, and therefore the spent-carrier problem is improved to some extent. However, even ferrite carriers are not satisfactory when used for developers in high speed copiers or printers in which developing sleeves or magnets in the sleeves rotate at high speed. In addition, carriers in which a variety of resins are coated on the surface thereof have been disclosed, however the resultant carriers cannot solve this problem.
Materials whose surfaces have low critical surface tension, for example, polytetrafluoroethylene and silicone resins, have been proposed as coating resins for the surface of the carrier. Although polytetrafluoroethylene can improve the spent carrier problem, it has a drawback that it cannot be used for a developer in which a toner is desired to be negatively charged because it has large negative polarity. Silicone resins can also improve the spent-carrier problem, however, they have a drawback in that a carrier having surfaces coated with silicone resins have a large resistivity which causes the so-called edge effect (a phenomenon that image density of a center part of a large solid image is lighter than that of an edge part thereof) in developed images, resulting in deterioration of the reproducibility of solid images and half-tone images. In addition, carrier particles having surfaces coated with silicone resins have a large counter charge when toner particles are released from the carrier particles, and, therefore, the carrier particles tend to be attracted to latent images particularly when a high bias voltage is applied for developing latent images.
In attempting to solve these problems, carriers having surfaces coated with a silicone resin including an electroconductive material such as carbon black have been disclosed, for example Japanese Laid-Open Patent Publication No. 56-126843 and a carrier whose surface is coated with a resin including porous carbon black, Japanese Laid-Open Patent Publication No. 62-45984. However, these carriers have a drawback that the carbon black in the coating layers of the carriers tends to release from the coating layers when the carriers are mixed with the toner or when developers including the carrier are continuously used for developing latent images, resulting in contamination of developing sleeves, photoconductors or the entire inside of the copy machine.
In addition, when these carriers are used for developers including a color toner, the released carbon black and the color toner are mixed, resulting in occurrence of a problem in which a developed image becomes a dark color. In attempting to solve this dark color image problem, a carrier having a coating layer including a white metal oxide has been proposed. For example, Japanese Laid-Open Patent Publication No. 64-35561 discloses a carrier having a silicone resin coating layer including one or more metal oxides of titanium oxide, zinc oxide and tin oxide. When a silicone resin including these conductive materials is coated on the surface of a ferrite type carrier, the spent-carrier problem can be improved and resistivity of the carrier can be easily varied, however high copy qualities, such as reproducibility of photographs or fine lines, which are now desired in the marketplace, cannot be obtained yet.
In order to achieve high copy qualities, the particle sizes of toner and carrier become finer and finer. When conductive materials are included in the coating layers of such fine carriers, they should have such a fine particle size as carbon black, and therefore need an additional pulverizing process of the conductive materials to make the coated carrier, resulting in higher manufacturing costs. When these fine conductive materials are included in a carrier coating liquid, the viscosity of the coating liquid seriously increases and therefore a problem occurs in which the coating cannot be performed by the spray coating method. Therefore carriers whose surfaces are coated with a thin resin layer without including these conductive materials are disclosed in, for example, Japanese Laid-Open Patent Publications No. 4-40472, 3-233464 and 7104522. These carriers have a drawback in that carriers coated with a thin resin layer do not have low resistivity, or if a carrier having low resistivity can be obtained, durability of the carrier deteriorates when used for continuous duplication.
Further, a coated carrier in which only a projected part of the carrier is not coated with a resin is disclosed in Japanese Laid-Open Patent Publication No. 4-93954, however it is difficult to obtain such carriers by the method proposed therein such as a fluidized bed type coating method, and if the carriers can be obtained, toner particles tend to adhere to the projected parts, resulting in occurrence of the spent-carrier problem.
Because of these reasons, a need exists for a carrier useful for a developer capable of developing electrostatic latent images which has low volume resistivity, high durability and which can produce images haying good image qualifies without undesirably making color images dark.